Recently, I’ve been asked to repair exterior window sills that have rotted. These sills are in window units that are 15-25 years old and were produced by major window manufacturers. Here in Maine as elsewhere in New England, architectural styles often try to reflect the traditional look of old windows from Georgian capes and colonials. This includes sills about 1-1/2″ thick and protruding an inch or two. The old windows were constructed of eastern white pine heartwood which is quite rot resistant. The new reproduction type window frequently contained softwoods that were less resistant to rot. Rotting sills have become common and in the past two years I have replaced approximately 25.
I start by removing the interior trim and stool cap. Then I pull the nails on the exterior casing and remove the unit. I then disassemble the unit and remove the offending pieces; usually the sill and one or both of the side casings. I then copy them using 100% eastern white pine heartwood. These pieces are primed, painted and re-installed with the appropriate caulking adhesive. I re-install the window unit and clean up. With normal maintenance these sills should last as long as the sills on the old capes and colonials.